The invention relates to a control unit and a method for preventing an undesired vehicle motion during an automatic switch-off process of a drive machine in a motor vehicle.
At the present time, internal combustion engines are normally switched off manually by the driver by means of an ignition key. One exception is represented by a method which is presented in DE 100 23 331 A1. Here, instead of the ignition key, the brake pedal position or the brake pedal pressure is evaluated. The switch-off process of the internal combustion engine is initiated when, after the motor vehicle has reached a standstill, the brake pedal is actuated more forcefully in its already actuated position. In order to travel onward, the brake pedal is released and the gas pedal is pressed, after which a start of the internal combustion engine is initiated. The disadvantage with such a method is that only the motor vehicle driver is responsible for switching off the internal combustion engine. Analyses of the current driving behavior show that, despite an increase in environmental awareness and increased fuel prices, switching off the internal combustion engine manually, for example at traffic signals, is seldom carried out independently.
In order to reduce fuel consumption and pollutant emissions, systems are already used in vehicles which automatically switch off the internal combustion engine of a motor vehicle under specific preconditions or in the presence of predefined switch-off conditions and switch on automatically again when predefined switch-on conditions are present. Methods and systems of this type and start-stop devices are primarily suitable for city traffic for reducing the fuel consumption since, in city traffic, the vehicle often comes to a standstill at traffic signals or because of the traffic, and the operation of the internal combustion engine is not required.
At the present time, individual vehicle manufacturers utilize various switch-off and switch-on logics with different advantages. For example, there are vehicles with automatic transmissions on the market in which the motor is switched off automatically when the driver has braked the vehicle to a standstill. The motor is started again as soon as the brake pedal is released. Another switch-off logic in automatic vehicles provides for the motor to be switched off only after about one second after a standstill has been reached.
Furthermore, DE 10 2008 061 790 A1 discloses an automatic switch-off logic for automatic vehicles having a delay circuit at a standstill, wherein, in addition, a holding function can be activated at the earliest after a standstill has been reached (=zero speed is measured). DE 10 2008 061 791 A1 reveals a similar method, wherein a so-called auto-hold function is activated when the brake pedal is released when the vehicle is at a standstill or when the vehicle has reached a standstill or when an automatic switch-off process is carried out.
Furthermore, DE 10 2010 000 615 A1 discloses an automatic switch-off logic for automatic vehicles, wherein the motor is switched off when the speed falls below a predefinable value (e.g. 20 km/h). If, in the further course, that is to say while the motor is off, a zero speed is measured (actual speed can be unequal to zero because of sensor inaccuracies), a braking control with successive increasing of the braking force to maintain the standstill is built up.
The object of the invention is, then, to provide a control unit and a corresponding method which prevent an undesired vehicle motion during an initiated automatic switch-off process of the drive machine, in particular when the switch-off process is discontinued.
According to the invention, this and other objects are achieved by a control unit and a method in accordance with embodiments of the invention.
The invention is based on the consideration of providing a switch-off logic in which the motor is stopped directly when the vehicle reaches a standstill, in order to conceal the shaking of the motor as it stops with a stopping jerk, i.e. the automatic switch-off process is already initiated shortly before the vehicle standstill is reached, so that the stopping of the motor coincides approximately with the vehicle reaching a standstill.
However, if, during the switch-off process, which is triggered with the brake pedal actuated, the driver releases the brake pedal because he wishes to travel onward (so-called change-of-mind situation) before the vehicle standstill has been reached, the vehicle is no longer secured against rolling back. The driver is possibly already actuating the accelerator and accordingly cannot himself also prevent the rollback.
Taking the above consideration into account, the first aspect of the invention is directed to a control unit for preventing an undesired vehicle motion during an automatic switch-off process of a drive machine in a motor vehicle, wherein an automatic switch-off process is initiated via a start-stop device before the standstill is reached if the motor vehicle is braked because of a deceleration request (on account of manual brake actuation or on account of automatic brake actuation from a driver assistance system), and in particular the speed of the vehicle is less than a predefined first (fixed or variable) speed threshold, so that the drive machine has been or is switched off approximately as the vehicle reaches a standstill (or when a vehicle standstill is detected). According to the invention, the control unit comprises
a module for acquiring a first signal indicating that the vehicle standstill will soon be reached,
a module for acquiring a second signal indicating that the inability of the drive machine to operate in a self-sustaining manner will soon be reached because of an initiated automatic switch-off process of the drive machine, and
a module for causing a brake pressure of a suitable level (that is to say a variably predefinable brake pressure) in the braking system to be locked in when the first and second signal are acquired.
If, therefore, it is established that, because of an automatically initiated shut-off process, it is anticipated that the motor will shortly no longer be able to operate in a self-sustaining manner (that is to say cannot start again without an appropriate starting device) and the vehicle standstill is imminent (but has not yet been reached), the brake pressure is locked in at a suitable level even before the vehicle standstill is reached and this pressure is maintained until the subsequent automatic switch-on process of the drive machine has been completed and a drive torque is provided at such a level that the vehicle can no longer roll backward or is again travelling forward. The vehicle is thus braked at a safe standstill, specifically even if the driver himself is no longer pressing on the brake pedal or a corresponding deceleration request is present. The imminent reaching of the vehicle standstill can be established, for example, if, by using evaluated signals, it is detected or suspected that the vehicle standstill will be reached within a defined time window, for example within about 500 ms.
As a further safeguard, a third signal relating to the speed falling below a predefined second speed threshold can advantageously be acquired by appropriate measures, and locking-in of the brake pressure at a suitable level can be caused only if, in addition to the two aforementioned signals, the third signal is acquired, if therefore the current vehicle speed is less than the predefined second speed threshold. This second speed threshold can be predefined fixedly at, for example, 3 km/h, or variably.
As a further safeguard, additionally or alternatively to evaluating the third signal by appropriate measures, a fourth signal relating to a predefined slope threshold of the roadway can advantageously be acquired, and locking-in of the brake pressure at a suitable level can be caused only if, in addition to the two aforementioned signals (and possibly the third signal), the fourth signal is also acquired, if therefore it is detected that the current roadway slope is greater than a predefined slope threshold. This condition is advantageous since, in particular in the case of greater slopes, there is a risk of rolling back when the brake is released. This slope threshold can be predefined fixedly at, for example, 1.5% slope, or variably.
Since the risk of rolling back generally exists only when the driver releases the brake pedal during the switch-off process, since he wishes to travel onward, the brake pedal actuation can additionally be monitored. For this purpose, measures can advantageously be provided which acquire a fifth signal relating to the brake pedal actuation, and locking-in of the brake pressure can then be caused at the latest when a release of the brake pedal or a released brake pedal is acquired.
In a particularly advantageous refinement of the invention, the level of the brake pressure to be locked in is taken from a characteristic curve, which predefines an appropriate braking torque as a function of the slope. More brake pressure is locked in, the greater the slope is. The corresponding brake pressure is then given by the frictional value of the brakes (so-called cp value).
Advantageously, the locked-in brake pressure is withdrawn when an undesired vehicle motion is no longer possible. Since, in the so-called change-of-mind case, this is generally no longer to be feared when a sufficient drive torque can be provided, the locked-in brake pressure is advantageously cancelled or withdrawn only when a sixth signal relating to a defined drive torque of the drive machine being exceeded is acquired. Alternatively, the locked-in brake pressure can also be withdrawn when the vehicle can be kept at a standstill on the basis of other actuators.
In a manner analogous to the control unit according to the invention, a second aspect of the invention is directed to a corresponding method for preventing an undesired vehicle motion during an automatic switch-off process of a drive machine in a motor vehicle, wherein, via a start-stop device, an automatic switch-off process is initiated before the standstill is reached if the motor vehicle is braked because of a (manual) deceleration request and, in particular, the speed of the vehicle is less than a predefined first speed threshold, so that the drive machine has been or is switched off approximately as the vehicle standstill is reached. The method according to the invention further provides that a first signal indicating that the vehicle standstill will soon be reached and a second signal indicating that the inability of the drive machine to operate in a self-sustaining manner will soon be reached because of an initiated automatic switch-off process of the drive machine can be acquired. If the first and second signal are acquired, a brake pressure of suitable level in the braking system will be caused to be locked in.
The above explanations relating to the control unit of the invention according to the first aspect of the invention apply in a corresponding way to the method of the invention according to the second aspect of the invention.
The method according to the invention and advantageous refinements thereof can be carried out by means of a processor implemented algorithm or an appropriate subassembly arrangement in a control unit provided for the purpose, in particular in a brake control device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.